Organic electroluminescent display device includes a matrix of organic electroluminescent (EL) or light-emitting elements wherein those elements selected so as to constitute characters or symbols are driven to emit light, thereby displaying the information. The organic EL elements have the basic structure that a hole transporting material such as tetraphenyl diamine (TPD) is evaporated or otherwise deposited as a thin film on a transparent electrode of tin-doped indium oxide (ITO) serving as a hole injecting electrode, a fluorescent material such as aluminum quinolinol complex (Alq3) is deposited thereon as a light emitting layer, and an electrode of a metal having a low work function such as magnesium is formed thereon as an electron injecting electrode. These organic EL elements are of great interest in the art because they can emit light at a very high luminance of 100 to 1,000 cd/m.sup.2 with a drive voltage of about 10 volts.
The organic EL elements have the problem that they are very sensitive to moisture. For example, the presence of moisture can cause separation between the light emitting layer and the electrode layers or degradation of the constituent materials, generating dark spots or failing to maintain light emission.
One method for solving this problem is disclosed in JP-A 89959/1993 wherein a gastight casing having a filling port is closely secured to a substrate so as to cover an organic EL multilayer structure on the substrate, and the interior of the casing is then filled with an inert gas. More particularly, after the gastight casing is closely secured to the substrate, the casing interior is evacuated through the filling port to remove residual moisture from the casing interior. An inert gas is introduced into the casing interior through the filling port whereupon the filling port is blocked, thereby establishing an inert gas atmosphere with a minimal water content in the casing interior for protecting the organic EL multilayer structure. In practice, the evacuation and inert gas introduction steps are repeated several times until the water content in the casing interior is reduced to an aim level.
The method of using such a gastight casing is successful in protecting the organic EL multilayer structure from moisture to some extent, but has the following problems. In a typical prior art system for manufacturing organic EL display devices, a first section for forming layers of the organic EL multilayer structure and a second section for introducing the inert gas into the casing joined to the organic EL multilayer structure are separate so that the layer forming operation and the gas filling operation are carried out separately. When the organic EL multilayer structure is transferred from the first section to the second section, there is a likelihood that the organic EL multilayer structure be exposed to the ambient air and moisture deposit on the organic EL multilayer structure. This moisture has a potential to later exacerbate the organic EL multilayer structure.
Further, when the gastight casing is closely secured to the substrate, epoxy resin base adhesives having high moisture resistance are typically used. Two-part mix type epoxy resin adhesives consisting of a base component part and a curing agent which are mixed on use, and thermosetting epoxy resin adhesives requiring no mixing are now commercially available. The two-part mix type epoxy resin adhesives have the advantage that they are curable at room temperature, but are less practical because of slow curing requiring several hours, a need for mixing before the application of adhesive, and a short pot life. On the other hand, the thermosetting epoxy resin adhesives are usually cured at as high a temperature as 140 to 180.degree. C. In the case of organic EL display devices which are manufactured by forming an organic EL multilayer structure on a substrate and then closely securing a gastight casing to the substrate with an adhesive, the heat resistance of organic EL multilayer structure becomes the bar against the use of thermosetting epoxy resin adhesives. It is well known that the heat resistance of an organic material as used in the organic EL multilayer structure is closely related to its glass transition temperature. There is no problem if the glass transition temperature of the organic material is higher than the curing temperature of thermosetting epoxy resin adhesives. However, most organic materials heretofore known suitable in the organic EL multilayer structure have a glass transition temperature of about 75.degree. C. to about 100.degree. C., and even special advanced organic materials with improved heat resistance have reached a glass transition temperature of about 130.degree. C. Therefore, the use of ordinary thermosetting epoxy resin adhesives is undesirable because the organic EL display devices can be thermally damaged.
There is a demand for adhesives which quickly cure without a need for heat. Photo-curing adhesives might satisfy these requirements. However, most photo-curing adhesives are acrylic adhesives of the radical curing type, which are inferior in moisture resistance to the epoxy resin adhesives. Additionally, acrylic monomers serving as an active ingredient of these adhesives can penetrate into or chemically react with the organic materials of the organic EL multilayer structure, degrading the structure to a sufficient extent to cause a failure or separation. Such degradation will result in non-light-emitting spots known as dark spots and shorten the luminous life. There are also available photo-curing/thermosetting combined type epoxy resin adhesives. Since these adhesives are obtained by the mixing or modification of acrylic adhesives of the radical curing type with thermosetting epoxy resin adhesives, they have not solved the above-mentioned problems.
The prior art inert gas introducing apparatus has the problems that every time when devices are manufactured, the steps of vacuum evacuation and inert gas introduction must be repeated several times for each device, and the filling port must be blocked, which lead to an increased number of steps and an increased manufacturing time.